In a pressurized nuclear powered steam generation system, water is passed over the reactor core in a pressurized vessel, where water is heated. From the core, the heated water is supplied to a steam generation vessel through piping referred to as the "hot-leg". In the steam generation vessel, the hot-leg water heats water that is converted to steam. The steam is employed to drive a steam turbine, producing electricity. The hot-leg water, cooled in the process, is returned to the reactor vessel in piping referred to as "cold-leg". Large pumps, mounted in the cold-leg piping, return the water from the steam generator to the reactor.
Servicing the pumps can present unusual problems. The pumps are very large and heavy, and water must be drained from the cold-leg to permit pump removal. The radioactive core must be furnished with cooling water during this period, otherwise it must be removed to an area where is can be cooled, a time consuming process during which the plant does not produce any electricity, increasing operating costs, especially when unplanned pump maintenance is needed. U.S. Pat. No. 4,957,693 shows apparatus that recirculates cooling water for residual heat removal (RHR) with a nuclear reactor system.